A wonderful punchy spice with a unique, nose-tingling pungency.
As you probably know, mustard seeds are tiny, round, hard seeds which can be black, brown or white (they actually come from different plants). The black seeds are extremely pungent and as they're difficult to harvest, they're more expensive. The white mustard seeds are milder but still pack a peppery punch. But it's really the black or brown mustard seeds that are most widely used in Indian cooking, and are more prevelant in southern Indian and Bengali food.
Mustard seeds should really be added at the start of the cooking process by dropping them into hot oil to release their unique peppery flavour. The seeds also give a deliciously nutty texture to most recipes, and are great in dry vegetable dishes as they add extra bite to the base sauce. But if you're looking for a spice combination that will blow your mind, then temper mustard and cumin seeds together as this creates an incredibly vibrant and peppery fusion of flavours.
Foody fact: in Eastern and Bengladeshi cuisine mustard seeds form part of the Panch Pooran spice blend. Health fact: these seeds contain high levels of magnesium which can help lower blood pressure and even help with the breaking down and digestion of your food.